Combined iron and B12/folate deficiency causes a rise in RDW.

Anemia resulting from concurrent nutritional deficiencies—specifically iron alongside vitamin B12 or folate—presents a complex hematologic profile known as dimorphic anemia. This condition significantly alters red blood cell (RBC) morphology and automated indices.

Clinical and effectiveness evidence

In cases of combined deficiency, the standard hematologic markers may be misleading if interpreted in isolation. While individual deficiencies typically push the Mean Corpuscular Volume (MCV) in opposite directions—iron deficiency toward microcytosis (<80 fL) and B12/folate deficiency toward macrocytosis (>100 fL)—their coexistence often results in a "pseudo-normal" MCV.

• RDW Sensitivity: Red Cell Distribution Width (RDW) is a highly sensitive marker for these mixed patterns, as it quantifies the variation in cell size (anisocytosis).
• Diagnostic Accuracy: Studies show up to 88.9% concordance between elevated RDW and manual peripheral blood smear findings.
• Screening Value: Because the MCV can be mathematically averaged into the normal range (80–100 fL) by the two competing cell populations, RDW serves as a critical diagnostic flag that further investigation (such as a peripheral smear or nutrient panels) is required.

Mechanistic explanations

The rise in RDW is a direct consequence of disrupted erythropoiesis at two different stages of cell development.

• Iron Depletion: Impairs heme synthesis, leading to the production of small, pale microcytes.
• B12/Folate Constraint: Impairs DNA synthesis and nuclear maturation, leading to larger macrocytes.
• Anisocytosis: The bone marrow simultaneously releases these two distinct populations into the circulation. RDW rises because it measures the standard deviation or coefficient of variation of the RBC volume distribution. A broad or bimodal curve on an automated RBC histogram reflects this high degree of size heterogeneity.

Bottom line

The claim is strongly supported by hematologic science: combined iron and B12/folate deficiencies create a mixed anemia pattern where RDW rises due to increased variability in red blood cell size. This elevation in RDW is often the primary indicator of deficiency when the MCV remains deceptively normal.